1. Field of the Invention
The present invention relates to restorative dentistry; specifically, dental implants relating to restorative and prosthetic dentistry.
2. Background of the Invention
Implants are now a standard way to attach a dental prostheses. One fixture may support a single tooth replacement, usually cemented or screwed atop an abutment. An implant supported bridge (also called a bar or frame) is used when several teeth are missing.
FIGS. 1A and 1B show the basic anatomical structure for a tooth, and a comparison between this structure and the structure most commonly used for a non-removable dental implant. Referring to FIG. 1A, the crown of the tooth includes an outer enamel layer. Beneath the enamel layer is the dentine and then pulp layer. The zone between the crown and the root portion of the tooth is known as the Cemento-Enamel-Junction (CEJ). The gingival tissue or gum surrounds the tooth. FIG. 1B shows the components of a typical single tooth implant juxtaposed with elements of a natural tooth. The implant includes the fixture (called an implant screw in FIG. 1B) and the prosthetic (abutment and crown).
The implant process begins with a determination that a prosthesis is needed to replace a tooth that is no longer capable of carrying chewing loads, no longer capable of supporting an artificial crown, or where the tooth is missing. The restorative dentist may consult with the oral surgeon, trained general dentist, prosthodontist or periodontist to co-treat a patient. Usually, physical models and/or impressions of the patient's jawbones and teeth are made by the restorative dentist at the surgeon's request, and are used as physical aids to treatment planning. If not supplied, the surgeon makes his own or relies upon advanced computer-assisted tomography or a cone beam CAT scan to arrive at a treatment plan.
The area in which the fixture is needed is examined by an oral surgeon who determines where in the patient's jaw a fixture can be safely supported by the bone. Conventional dental x-rays are sometimes relied on to learn if the underlying bone structure appears suitable to support implants and to identify the areas where nerves or other vital anatomical structures are located. There must be bone having a sufficient load-carrying capacity, i.e., a bone having sufficient bone density and adequate depth and width to support normal and transverse loads on the implant. If bone volume or density is inadequate, a bone graft procedure must be considered first.
Unaided manual preparation of a jaw for fixtures supporting prosthesis is challenging, because of the difficulty in estimating positions and angles accurately by the naked eye, within a deep hole of small diameter in a patient's mouth. Even if the work is being done by an experienced dentist or oral surgeon, chances for location, angular or orientation errors are great. For this reasons, drill guides are needed to assist with locating not only the proper drill depth, but entry angle of the drill. Positioning or depth indicators have also been developed to assist with obtaining the appropriate depth and orientation of the hole that will receive the fixture. This part of the process, however, is largely if not wholly controlled by the oral surgeon's determination of how to best hold the fixture in the existing bone, avoiding nerve endings, etc. In other words, the oral surgeon's selection of the type and size of the hole needed, the corresponding fixture screw size, its pitch, diameter, and orientation is not also constrained or a function of the patient's bite or the bite registration, the external loading on the prosthesis for the patient's particular mouth, e.g., the orientation of the adjacent teeth or how they will ultimately function in connection with the adjacent prosthesis, or the nature of the soft tissue surrounding the fixture sight. The oral surgeon drills and places the fixture simply based on the location of bone capable of safely supporting the fixture.
A custom drill guide is now often created to help guide the oral surgeon's drill. Cone Beam technology is used to capture an enhanced view of the upper and lower jaw region of a patient's head. The resulting imagery can show the bone structure and teeth in detail as well as the soft tissues. Using specially designed software that aids in predefining appropriate fixture locations, the Cone Beam data can be used to create another set of data defining the location, orientation, and depth of each cavity to be prepared. From this, with use of a numerically controlled drilling tool, a patient- and case-customized drill guide or surgical guide is constructed. When properly mounted in the patient's mouth, guided holes in this unit align the drilling tool for its use in creating each predefined fixture cavity. Each fixture is then inserted and moved into its permanent location.
After installation of a fixture screw, the implant planning and installation can vary, depending on how long a delay (of up to six months) is allowed for accommodation of the fixture(s) by the bone of the jaw. Some fixture manufacturers recommend loading fixtures immediately, others do not. If a healing delay is to be observed, a healing abutment or a cover screw—a metal extension washer with a domelike-top—is fastened to each fixture by a screw in the threaded hole of the fixture, and the gum flesh is sutured over the abutment.
On successful completion of the foregoing fixture procedure, the patient returns to the Dentist for the later process steps. To install the prosthesis, tissue over the fixture is reopened using a knife or a punch. The healing abutment or the cover screw is removed from the fixtures to reveal the surfaces on which the frame's attachment points will rest. Dental impressions are made of upper and lower jaws using transfer metal copings that attach to the fixture level of the implant. Molds (positive models of the jaws) are made from these impressions, in a traditional procedure duplicating the position of the implants, the soft tissue and the natural teeth. The dental impression or physical molds after being shipped to a dental laboratory are used to build up a prosthesis, in a traditional highly labor-intensive process demanding high accuracy, skill level and long experience for good results.
Thus, traditional prosthesis planning begins after the fixture is installed, not at the beginning, before any surgery has taken place. The traditional process may be likened to that of a house built in an ad-hoc fashion. The ground is excavated and cement poured to create a supporting formwork for a building before deciding what type of building will be supported by the basement, the environmental conditions that the building must withstand, or how the building will sit relative to adjacent architecture. It would be preferred to arrive at a whole design of the integrated prosthesis (fixture, abutment and crown) from the beginning, before any surgery has taken place so that the best implant for the job can be fashioned. In order to do so, the collective sum of the knowledge that goes into each step of creating and installing a prosthesis should be considered.
Other suggestions for implant planning and selection, and related concepts are described in U.S. Pub. No. 2007/0154866, U.S. Pat. No. 7,322,824 and U.S. Pub. No. 2008/0153061
In view of the above, it will be appreciated that today's typical protocol for preparation of the mouth for, and placement of, dental implants involves the following considerations:
a) The human jawbone is highly variable in thickness and density from location to location, and varies from person to person. Thus, for a given individual's jaw, certain implant locations are preferable to others because of bone strength variations.
b) For implant attachment strength, the optimal direction at which the fixture should pass into the bone varies from one jaw location to another, and bone configurations are different from person to person. If the hole in the bone is drilled at an incorrect location and/or angle, the tip of the fixture may pass through the bone and out the far side, weakening its attachment strength and in some instances compromising the integrity of the entire fixture. Protruding fixture tips also raise patient objections on cosmetic grounds.
c) Poor placement of fixtures can be a source of problems in installing and using a prosthesis. If fixtures exit the jaw unparallel with one another it may be more difficult to align the prosthesis to the fixtures properly. In addition, when fixture axes are far from parallel, biting forces will translate from purely compressive force to bending force more likely to fracture the bone, the fixture itself or the prosthetic screws holding the prosthesis to the fixtures.
The known art for the fixture process usually includes installing a titanium screw, installing an abutment, and then installing a corresponding crown atop the abutment. Safety and aesthetics are usually considered during this process (as noted above), but due to a lack of an available systematic analysis of the overall restorative device functions after implantation, the fixture may not function as intended. This may lead to subsequent return trips to the restorative dentist or surgeon replacement of crowns or repair of the supporting jaw due to extensive bone loss, infections, etc.
It would be preferred to have answers to questions such as the functional aspect of the final implant restoration from the implant tip representing the root tip of the natural tooth to the cusp tip of the fabricated crown and the final occlusion and how this effects proper placement of the implant, before the implant is placed in the mouth. For example, how much pressure is being placed on the bone-implant interface? Implant loads from chewing and parafunction can exceed the physio biomechanic tolerance of the implant bone interface and/or the titanium material itself, causing failure. This can be a failure of the implant itself (fracture) or bone loss, or a “melting” or resorption of the surrounding bone.